COMEDK2025ChemistryThermodynamics (C)Actual
The enthalpies of combustion of H ₂, C (graphite) and C ₂ H ₆( ~g ) are -286.0,-394.0 and -1560.0 ~kJ ~mol ⁻¹ at 25^ C and 1 atm pressure. The enthalpy of formation of ethane is:
Options
- A-97.0 ~kJ ~mol ⁻¹
- B-78.0 ~kJ ~mol ⁻¹
- C-86.0 ~kJ ~mol ⁻¹
- D-92.0 ~kJ ~mol ⁻¹
Correct answer
C. -86.0 ~kJ ~mol ⁻¹
Step-by-step solution
The combustion reactions for the given substances are: 1) H ₂(g) + 1 2 O ₂(g) H ₂ O (l) , H₁ = -286.0 kJ mol ⁻¹ 2) C (s) + O ₂(g) CO ₂(g) , H₂ = -394.0 kJ mol ⁻¹ 3) C ₂ H ₆(g) + 7 2 O ₂(g) 2 CO ₂(g) + 3 H ₂ O (l) , H₃ = -1560.0 kJ mol ⁻¹ The formation reaction for ethane is: 2 C (s) + 3 H ₂(g) C ₂ H ₆(g) , H_f = ? Using Hess's Law, we can express the formation reaction as: H_f = 2 ( H₂) + 3 ( H₁) - ( H₃) Substituting the given values: H_f = 2(-394.0) + 3(-286.0) - (-1560.0) H_f = -788.0 - 858.0 + 1560.0 H_f = -1646